31-Mix Snap-On Ferrite

If you're in need of a mains filter or ad-hoc RF choke, this snap-on 31-mix ferrite is for you.  With increased amounts of domestic "QRM" such as switch-mode power-supplies, televisions, ADSL routers and LED lighting making HF reception in some situations a non-starter - how do you go about taming it?

Known as the Problem Solver in GM3SEK's In Practice articles (and a live demo in this RSGB "Clean Up Your Shack" video) the snap-on design enables you to wind several turns of cable in a non-invasive manner.  31-mix is perfect for use across HF and especially on the lower bands - and more than 3x thicker than a typical FT240-31 toroid, it's a bit of an animal.

• Fair-Rite 0431177081, 31-mix material
• 56mm length, 43mm width
• Internal diameter: 25.6mm - Can take 4 turns of RG213

As a Mains Filter: The picture above shows how the conductors from a 2.5mm twin&earth cable form 5 turns in total.  You may wish to experiment with the placement of the filter as it's designed to stop the mains wiring acting as an aerial for any "nasty" electrical devices.  Can be wound on extension cables or as part of the "twin and earth" wiring detailed above.

It's also wise to test your house without mains power - Switch-off the main circuit-breaker and run your radio off a battery: How is the noise-floor now?  If it has dropped, then that's good news - it means that you can switch things back on one at a time to discover what is causing the interference.  With some time and a little patience, you may get a few S-points back and possibly a much improved HF experience.

As an RF Choke : 4-8 turns should be sufficient across the HF bands.  This is a great temporary solution if you need to find the best place to fit a choke on your feeder before committing to one of our Balun/Choke boxes.  We also sell 31-mix ferrites in FT240 (2.4") diameter for Baluns, Chokes etc.  Smaller 5mm snap-on , 9mm snap-on and 13mm snap-on ferrites for USB and coaxial cables also available.

When it comes to RFI issues, experimentation is key!

There is no guarantee that any ferrite will cure noise on your radio, and it may not be possible to "filter" any noise from your neighbour's equipment that is radiating in the air (eg: solar-panels, greenhouse/fish-pond equipment etc).  "Ferrite-on-cable" is the best approach, and significant gains can usually be obtained by getting your own house in order before tackling external sources.  Experimentation with placement, turns and quantity of ferrites is key - there is never a one-size-fits-all solution!